Two desirable attributes of lasers operable in the visible and infrared are high output power and tunable output wavelength. These attributes are available individually--existing carbon dioxide lasers are capable of providing 10 kilowatts output at a fixed wavelength, and there exist tunable lasers capable of providing variable visible wavelengths at output powers ranging from milliwatts up to approximately 2 watts. However, there have not heretofore been any lasers capable of providing both of the sought after attributes in a single device.
The free electron laser as described in U.S. Pat. No. 3,822,410 has shown considerable promise in making available a tunable high power device. By way of background, a free electron laser is a device which couples the kinetic energy of a relativistic electron into the field energy of a freely propagating plane wave. The electron beam is periodically deflected by a transverse magnetic field defined by a linear array of so-called "wiggler" magnets. Alternating magnets have opposing polarities, so that the beam follows an undulating or helical path. The induced transverse velocity allows the electron to do work on the electric field of the radiation. Each time the electron is deflected, it emits a burst of radiation and the combination of the individual bursts yields a beam of coherent radiation at a wavelength given by: ##EQU1## where: .mu..sub.0 is the wiggler magnet period;
.gamma. is the electron energy divided by its rest mass energy; PA1 B is the magnetic field strength of the wiggler magnets; PA1 e is the electron charge; PA1 m is the electron rest mass; and PA1 c is the speed of light; PA1 with e, B, m, c and .lambda..sub.0 being in cgs units.
In operating such a laser, the output wavelength is most conveniently tuned by varying the electron energy (proportional to .gamma.) or the magnetic field strength, and interesting levels of power output (say 100 watts) may be achieved. In a typical embodiment, the electrons are recirculated in a storage ring so as to make multiple passes through the laser stage. Such a device is known as a storage ring laser. An accelerator stage comprising an RF cavity in the storage ring restores to the electron beam the energy which was radiated as synchrotron radiation and that which was transferred to the electromagnetic wave in the laser.
The inherent nature of the storage ring is that the electrons move in a potential well (the RF "bucket") and undergo bunching on the order of a meter or so. The electrons are characterized by an equilibrium energy distribution that is a function of the storage ring. In operation, the laser field produces potential wells ("optical traps") so that the electron beam entering the laser magnet stage undergoes a further bunching on the scale of the optical wavelength. While a uniformly distributed beam (constrained only by the RF bucket) can only exchange energy to second order in the radiation field, a bunched beam can exchange energy to first order. However, one of the difficulties encountered in the design of a storage ring laser arises from the laser-induced heating of the electrons. The laser phase-dependent energy loss induces additional energy spread on the electron beam when the electrons enter the laser with random optical phase. The spread is typically greater than the (second-order) mean radiated energy, and the efficiency is limited by the dissipation which must be introduced to control the spread. Thus, the bunches formed on a given pass through the laser stage tend to be lost, so that on the next pass, a new bunching regime must be established. Accordingly, the output power in the laser stage has tended to be limited to but a few percent of the synchrotron radiated power in the storage ring.